In order for land managers and policy makers to manage excessive soil phosphorus (P) concentrations and reduce the risk of this particular source of P from impacting water bodies, models of soil P decline under various scenarios are needed. We modelled the decrease in calcium chloride-extractable P (CaCl2-P), and sodium bicarbonate-extractable P (Olsen-P and Colwell-P) using data from six Australian grazed pasture soils with contrasting P sorption properties, over a period of 4.5 years. Each soil had four initial soil P concentrations (Pinit), each of which received four on-going rates of P fertiliser (Pfert). The model predicts the final P concentration (Pfinal) by taking into account the P concentration previously measured (CaCl2-P, Olsen-P or Colwell-P), Pfert applied since measurement, and time since previous measurement: Final P concentration = (previously measured P concentration + ep x P fertiliser applied) exp (-dp x years since previous P concentration measurement). Where ep is the increase in soil P for each unit of applied P and dp is the decay constant representing how quickly the soil P decreased. The greatest decreases in proportion to Pinit occurred for CaCl2-P, followed by Olsen-P, and then Colwell-P. The model tended to fit the dataset well for Olsen-P and Colwell-P, with mean overestimation (modelled Pfinal concentration greater than actual Pfinal) of the Pfinal concentrations of 6.1 (32%) and 4.3 mg/kg (10%), respectively. Although there was less CaCl2-P data, the model successfully described it, with a mean overestimation of Pfinal CaCl2 of 3.1 mg/kg (26%). The overestimation of Pfinal CaCl2 was possibly due to the high CaCl2-P concentrations of the low P buffering index soils. The model predicted an average of 32 years (ranging from 26 to 49 years) for Olsen-P concentrations of between 55 and 96 mg/kg to decrease to an agronomic optimum of 17 mg/kg. Agronomic optimum was not a reliable indicator of environmental risk as some soils did not exceed the CaCl2-P environmental threshold until Olsen-P concentrations were twice the agronomic optimum, whereas low P sorbing soils tended to exceed the threshold before reaching agronomic optimum. Further work with more soils is required to examine the influence of soil properties – such as P sorption – on decreases in soil P.

中文翻译：

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当磷肥减少或停止时，模拟土壤磷的变化

为了让土地管理者和政策制定者管理过量的土壤磷 (P) 浓度并降低这一特定磷源影响水体的风险，需要各种情景下土壤磷下降的模型。我们使用来自六个澳大利亚放牧牧场土壤的数据模拟了氯化钙可提取的 P (CaCl2-P) 和碳酸氢钠可提取的 P（Olsen-P 和 Colwell-P）的减少，这些数据来自具有对比 P 吸附特性的 4.5 年年。每个土壤有四个初始土壤磷浓度 (Pinit)，每个土壤都接受四个持续的磷肥 (Pfert)。该模型通过考虑先前测量的 P 浓度（CaCl2-P、Olsen-P 或 Colwell-P）、自测量以来应用的 Pfert 以及自上次测量以来的时间来预测最终 P 浓度 (Pfinal)：最终 P 浓度 =（先前测量的 P 浓度 + ep x 施用的 P 肥料）exp（-dp x 自上次 P 浓度测量以来的年份）。其中 ep 是每单位施磷后土壤磷的增加量，dp 是衰减常数，表示土壤磷下降的速度。CaCl2-P 与 Pinit 的比例下降最大，其次是 Olsen-P，然后是 Colwell-P。该模型倾向于很好地拟合 Olsen-P 和 Colwell-P 的数据集，Pfinal 浓度的平均高估（模拟 Pfinal 浓度大于实际 Pfinal）分别为 6.1 (32%) 和 4.3 mg/kg (10%) . 尽管 CaCl2-P 数据较少，但该模型成功地描述了它，Pfinal CaCl2 的平均高估为 3.1 mg/kg (26%)。Pfinal CaCl2 的高估可能是由于低 P 缓冲指数土壤的高 CaCl2-P 浓度。该模型预测，Olsen-P 浓度在 55 至 96 mg/kg 之间的平均 32 年（范围为 26 至 49 年）会降至 17 mg/kg 的农艺最佳值。农艺优化不是环境风险的可靠指标，因为一些土壤在 Olsen-P 浓度是农艺优化的两倍之前不会超过 CaCl2-P 环境阈值，而低 P 吸附土壤在达到农艺优化之前往往会超过阈值。需要对更多土壤进行进一步研究，以检查土壤特性（如磷吸附）对土壤磷减少的影响。该模型预测，Olsen-P 浓度在 55 至 96 mg/kg 之间的平均 32 年（范围为 26 至 49 年）会降至 17 mg/kg 的农艺最佳值。农艺优化不是环境风险的可靠指标，因为一些土壤在 Olsen-P 浓度是农艺优化的两倍之前不会超过 CaCl2-P 环境阈值，而低 P 吸附土壤在达到农艺优化之前往往会超过阈值。需要对更多土壤进行进一步研究，以检查土壤特性（如磷吸附）对土壤磷减少的影响。该模型预测，Olsen-P 浓度在 55 至 96 mg/kg 之间的平均 32 年（范围为 26 至 49 年）会降至 17 mg/kg 的农艺最佳值。农艺优化不是环境风险的可靠指标，因为一些土壤在 Olsen-P 浓度是农艺优化的两倍之前不会超过 CaCl2-P 环境阈值，而低 P 吸附土壤在达到农艺优化之前往往会超过阈值。需要对更多土壤进行进一步研究，以检查土壤特性（如磷吸附）对土壤磷减少的影响。农艺优化不是环境风险的可靠指标，因为一些土壤在 Olsen-P 浓度是农艺优化的两倍之前不会超过 CaCl2-P 环境阈值，而低 P 吸附土壤在达到农艺优化之前往往会超过阈值。需要对更多土壤进行进一步研究，以检查土壤特性（如磷吸附）对土壤磷减少的影响。农艺优化不是环境风险的可靠指标，因为一些土壤在 Olsen-P 浓度是农艺优化的两倍之前不会超过 CaCl2-P 环境阈值，而低 P 吸附土壤在达到农艺优化之前往往会超过阈值。需要对更多土壤进行进一步研究，以检查土壤特性（如磷吸附）对土壤磷减少的影响。